1. Field of the Invention
This invention relates to direct view displays and more specifically to a paper white display that combines flat-panel addressing technology with a micromirror faceplate.
2. Description of the Related Art
Direct-view displays convert electrical signals into images that can be viewed directly without the aid of magnification or projection. The market for direct view displays spans a continuum of performance and price that includes the ultra high performance but very expensive flat-panel DTVs, moderately performing and priced laptop computers, and the lower performing but much cheaper personal digital assistants, electronic books and cellular telephones. The high end displays offer high spatial and color resolution but are very expensive and consume a lot of power. The low end displays offer less resolution but are relatively cheap and can be operated off of battery power.
The most common direct view technology is the cathode ray tube (CRT), in which a scanning electron gun shoots a single beam of electrons across a vacuum to scan a phosphor-coated anode. The electrons penetrate the individual phosphors causing them to emit light, which can be viewed directly or projected onto a screen. The brightness of CRT displays is inherently limited by the phosphors ability to emit light. In addition, many CRT displays decay to a half-brightness level after about one year of use.
By necessity, the gun in a CRT must sit far from the anode to raster scan the phosphor screen, a distance similar to the width of the display area As a result, high-resolution large area CRT displays are correspondingly very bully, and thus not suitable for portable displays. During the past 40 years numerous attempts have been made to construct a "Flat-CRT" for direct view applications, which can overcome the size and power limitations of the conventional CRT without sacrificing performance.
The Aiken thin CRT, or "Thintube," was invented in the 1950s and was produced in small quantities in the early 1960s. The Thintube uses a single electron gun positioned in the plane of the phosphor screen in conjunction with horizontal and vertical deflection plates that first turned the beam down the selected row and then turned the beam to address the phosphor screen at the selected column. The contrast, size and luminance achieved by the Thintube rival those achieved by current flat panel technologies. Phillips and Sharp are currently developing variants of the original Thintube technology.
Matrix addressed flat panel displays such as the Plasma Display Panel (PDP) and Field Emission Display (FED) are two of the more promising and established phosphor-based flat-CRT technologies. Emerging flat panel technologies include the surface conduction electron (SCE) array, the metal-insulator-metal (MIM) cathode array and the Magnetic Matrix Display (MMD) that are being pioneered by Canon, Hitachi and IBM, respectively, to drive a phosphor screen.
The PDP can be thought of as a set of matrix addressed neon bulbs. An AC or DC voltage applied across a small gap containing an emitting gas, e.g. 3-10% Xenon diluted in either Helium or Neon, causes the gas to ionize and emit ultra violet light, which in turns excites the RGB phosphors to produce visible light. PDPs have been fabricated as large display areas with wide viewing angles and fill-color performance but its lumens/watt efficiency is very poor. The low efficiency and relatively high cost limits the PDP to a few niche applications such as a wall-hanging displays with a diagonal of one meter or more.
The FED utilizes a matrix addressed cold cathode array, spacers to support the atmospheric pressure, and cathodoluminescent phosphors for efficient conversion of the electron beam into visible light. The non-linearity of the current/voltage relationship permits matrix addressing of high information content displays while providing high contrast ratio. The FED combines the best properties of CRTs (full color, fill grey scale, brightness, video rate speeds, wide viewing angle and wide temperature range) with the best attributes of Flat Panel technology (thin and light weight, linearity and color convergence). However, the current FEDs have limited display sizes, a 12 inch diagonal or less, due to the fabrication and vacuum packaging problems. Since the primary motivation for Flat-CRTs was to overcome the size and weight limitations of the conventional CRT for large display sizes, this is a serious problem to the successful commercialization of the FED technology.
The portable or laptop computer display market is dominated by liquid crystal display (LCD) technology and particularly the Active Matrix LCD (AMLCD) technology, also known as TFT (thin film transistor) displays. The a liquid crystal panel is fabricated with an array of thin film transistors, one per cell, that are driven by row and column drive electronics. The row drivers enable the transistors a row at a time while the column drivers apply selected voltages that are latched through the transistors to the respective cells. The voltage changes the transmissive characteristics of the liquid crystal, which in turn optically modulates the amount of light transmitted through the LCD. Liquid crystal imagers require the use of polarizers that absorb a large fraction of the light, typically 60%-70% and reduce optical efficiency. The TFT element and the electrical connections to it are made of opaque material. This further reduces the amount of transmitted light allowed by as much as 50% and also causes a pixelation effect.
Multiplexed LCDs were the precursor to AMLCDs. Multiplexed LCDs sacrifice gray scale performance in favor of fabrication simplicity and power consumption by eliminating the TFT array. Because liquid crystals respond relatively slowly to changes in the applied voltage, the cell modulation is proportional to the root-mean-square (rms) voltage applied across the cell throughout the frame time. Although the voltage applied during the row enable is very large, the background noise created by the applied voltages for the remaining n-1 rows greatly reduces the RMS value of the margin between the off-state and full on-state of the liquid crystal. For example, commercially available AMLCDs can resolve about 16 million different colors while similarly available multiplexed LCDs can resolve only 256 different colors. As the number of scanned rows increases, this disparity in grey scale color resolution grows.
As a result, multiplexed LCDs are used in applications such as low end laptop computers and personal digital assistants (PDAs). These applications demand low cost and lower power consumption but can tolerate reduced gray scale resolution. Although they consume less power than AMLCDs, the multiplexed LCDs still consume too much power for many portable applications. Regardless of whether the display supports video applications or only non-video applications, these LCDs must be constantly refreshed, e.g. 30 times per second. Without a sustaining voltage they will decay from their modulated state to their relaxed state over time. Furthermore, the polarizers inherently required by LCDs absorb such a large fraction of the ambient light, they are unable to produce the "paper white" quality desired by the industry. As such consumers must make do with cell phones and PDAs whose gray displays are difficult to read even under the best ambient lighting conditions. Power consuming backlights must be added to improve their readability to minimum acceptable levels.
A more recent modification to the basic AMLCD technology is the Plasma Addressed Liquid Crystal (PALC) display. PALC uses the electrical switching properties of an ionized gas to actively address the liquid crystal pixels. In the integrated LCD/plasma structure of PALC, each scan line is defined by a plasma channel. The cathodes in the channels are sequentially scanned by applying a plasma discharge voltage, resulting in all pixels in each scanned line being addressed in a line-at-a-time fashion. PALC has the potential to exhibit high image quality, large display sizes and low manufacturing costs. Whereas AMLCDs use traditional integrated circuit lithography, PALC is more similar to printed circuit board lithography. Printed circuit boards can be made much larger and at lower costs than ICs.
A number of electromechanical shutter display technologies have been pursued and patented for direct view displays, but have not succeeded to large scale commercialization due to a variety of issues including fabrication, stiction, limited contrast ratio, poor optical efficiency, high cost and poor pixel uniformity.
U.S. Pat. No. 5,552,925 to Worley entitled "Electro-Micro-Mechanical Shutters on Transparent Substrates" combines micro-mechanical and silicon-on-transparent-substrate technologies to construct a transmissive micro-mechanical shutter array that is actuated by electrostatic forces in either an electric force/electric counter-force or electric force/mechanical counter-force configuration. In both configurations, the shutter array is addressed using an active matrix similar to that used in AMLCDs and thus will exhibit the same optical throughput and pixelation problems. Retention capacitors are used to hold the voltages across the shutters until the next frame.
Referring now to Worleys FIGS. 5 and 5A, there is shown a "flapper" type micro light shutter that comprises a flapper, a select MOSFET transistor, a row access line, and a column signal line formed on a substrate. Located above the shutter is a light mask comprised of a transparent substrate, a transparent ground electrode and an opaque blocking layer that shields the MOSFET transistor. When a voltage is applied between the flapper and an overhead ground plane, the electric force created pulls the flapper toward the plane. As the flapper moves in response to the electric force, a counter-force is provided by the spring constant of the flapper. As the voltage differential increases, the angle the flapper makes with the substrate increases, which in turn allows more light to pass through the transparent substrate. Because of the very low open aperture of this design, the optical throughput of this structure is too low to be commercially viable.
U.S. Pat. No. 3,553,364 to Lee entitled "Electromechanical Light Valve" describes an electromechanical light valve in an array of many such valves for controlling the transmission of light in continuously changing patterns. Each light valve consists of a housing having grounded conducting walls for shielding the interior thereof from external electrostatic forces produced by surrounding valves and a leaf shutter mounted in the housing. The application of a voltage to the leaf shutters causes the shutter to be attracted to the grounded conducting walls. As the voltage differential increases, the angle the shutter deflects increases, which in turn allows less light to pass through the housing.
Lee's design always involves the leaf shutters touching one surface or another, e.g. the conductive center plate or the grounded conductive walls, which can and will cause stiction due to the Van der Waals forces. Because Lee's design switches between these two surfaces, the leaf shutter will experience the non-linear pull-in effect, which precludes a controllable gray scale. The optical efficiency of this design is also very low on account of the low open aperture due to the opaque conductive sidewalls. In addition, the cost and complexity of fabricating an array of such housings makes high resolution displays impractical.
U.S. Pat. No. 4,564,836 to Vuilleumier et al. entitled "Miniature Shutter Type Display Device with Multiplexing Capability" describes a display device comprising an insulating carrier and shutters that are capable of rotating under the effect of an electric field. The shutters are grouped in pairs and are controlled by applying a voltage between the shutter and a counter-electrode. A holding voltage is then applied between the pair of shutters to hold them in place. Vuilleumier's device has no gray scale capability and involves shutters touching each other or a stop which can cause stiction problems. This design also has low optical efficiency due to the opaque sidewalls of the individual cavities.
U.S. Pat. No. 5,784,189 to Bozler et al. entitled "Spatial Light Modulator" discloses a spatial light modulator formed of a moveable electrode which is disposed opposite a fixed electrode, and is biased to roll in a preferred direction upon application of an electric field across the electrodes to produce a light valve or light shutter. As shown in FIG. 9, a reflection mode device is disclosed that uses a white background and black shutter coils. In one embodiment, the moveable electrode is a coiled electrode fixed at one end, which rolls up in a preferred direction and unrolls upon application of an electric field across the electrodes. In another embodiment shown in FIGS. 32-35, the moveable electrode is a hinged shutter having a bowl-like shape with two comers that contact electrical conductors and two comers that contact electrical insulators. The Van der Wall's forces between the comers and the electrical conductors are inherently stronger than the forces to the insulators. Therefore when a proper force is applied to the shutter, the insulator contacts will break free and the shutter will rotate about an axis passing through the two conducting contacts.